Multilayer ceramic capacitors have often been operated with low-frequency low-voltage alternating current or low-voltage direct current. In recent years, however, with developments in electronics, as rapid progress has been made in the miniaturization of electronic devices, multilayer ceramic capacitors also have been reduced in size and increased in capacity. As a result, a voltage applied between a pair of electrodes in such a multilayer ceramic capacitor has tended to relatively increase. Such multilayer ceramic capacitors have been required to have higher capacitance, lower loss, improved insulation properties, improved dielectric strength, and higher reliability under stringent conditions.
For example, Patent Documents 1, 2, and 3 propose dielectric ceramic compositions and multilayer ceramic capacitors that can be used with high-frequency, high-voltage alternating current or high-voltage direct current.
The dielectric ceramic composition described in Patent Document 1 is represented by a general formula ABO3+aR+bM (wherein ABO3 is a general formula representing a barium titanate solid solution; R represents an oxide of at least one element selected from La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu; M represents an oxide of at least one element selected from Mn, Ni, Mg, Fe, Al, Cr, and Zn), wherein A/B, a, and b in a main component meet the following relationships 0.950≦A/B≦1.050, 0.12≦a≦0.30, and 0.04≦b≦0.30. The dielectric ceramic composition contains 0.8 to 8.0 parts by weight of a sintering additive as an accessory component with respect to 100 parts by weight of the main component. The dielectric ceramic composition may further contain 0.35 mol or less of X(Zr,Hf)O3 (wherein X represents at least one element selected from Ba, Sr, and Ca) with respect to 1 mol of the barium titanate solid solution and/or 0.2 mol or less of D (wherein D represents an oxide of at least one element selected from V, Nb, Ta, Mo, W, Y, and Sc) with respect to 1 mol of the barium titanate. The firing temperature of the dielectric ceramic composition is 1,300° C. or lower. The dielectric ceramic composition has a dielectric constant of 200 or more, low loss when operated with high-frequency, high-voltage alternating current, high insulation resistance at high electric strength, characteristics meeting the B and X7R characteristics, and excellent high-temperature load properties.
A reduction-resistant dielectric ceramic compact described in Patent Document 2 includes a solid solution mainly composed of barium titanate; and a sintering additive. The axis ratio, i.e., c/a, of the ceramic compact is determined by X-ray diffraction and meets 1.000≦c/a≦1.003 at a temperature of −25° C. or higher. When the dependence of the dielectric constant on temperature is measured with an alternating current having a frequency of 1 kHz at an electric field strength of 2 Vrms/mm, the maximum peak is observed at less than −25° C. The main component is represented by a general formula ABO3+aR+bM (wherein R represents an compound containing at least one element selected from La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu; and M represents a metal oxide containing at least one element selected from Mn, Ni, Mg, Fe, Al, Cr, and Zn), wherein A/B (molar ratio), a, and b in a main component meet the following relationships 1.000≦A/B≦1.035, 0.005≦a≦0.12, and 0.005≦b≦0.12. The dielectric ceramic composition contains 0.2 to 4.0 parts by weight of the sintering additive with respect to 100 parts by weight of the main component. The dielectric ceramic compact may further contain 0.20 mol or less of X(Zr,Hf)O3 (wherein X represents at least one element selected from Ba, Sr, and Ca) with respect to 1 mol of the barium titanate solid solution and/or 0.20 mol or less of D (wherein D represents an oxide of at least one element selected from V, Nb, Ta, Mo, W, Y, Sc, P, Al, and Fe) with respect to 1 mol of the barium titanate solid solution. The dielectric ceramic composition has low loss and low heat generation when high frequency/high voltage are applied and exhibits stable insulation resistance with DC/AC load.
A dielectric ceramic composition described in Patent Document 3 includes barium titanate, a rare-earth oxide (wherein the rare-earth element is at least one element selected from Y, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb), calcium oxide, and silicon oxide without magnesium oxide. The barium titanate is expressed as BamTiO3. The rare-earth oxide is expressed as RO3/2 (wherein R represents a rare-earth element. Calcium oxide is expressed as CaO. Silicon oxide is expressed as SiO2. The dielectric ceramic composition is represented by a general formula 100BamTiO3+aRO3/2+bCaO+cSiO2 (wherein coefficients 100, a, b, and c represent molar ratios), wherein m, a, b, and c meet the following relationships 0.990≦m≦1.030, 0.5≦a≦30, 0.5≦b≦30, and 0.5≦c≦30. The dielectric ceramic composition does not contain MgO because the coexistence of CaO and MgO causes a deterioration in reliability. The dielectric ceramic composition has the following advantages: Temperature characteristics thereof satisfy the B characteristics specified by JIS and the X7R characteristics specified by EIA. The dielectric loss is as small as 2.5% or less. The product (CR) of insulation resistance (R) and capacitance (C) is 10,000 Ω·F or more when 4 kVDC/mm is applied at room temperature. Insulation resistance is ensured for a prolonged period in accelerated life testing with high voltage at high temperature. Therefore, it is possible to form a highly reliable thinner dielectric ceramic layer for a multilayer ceramic capacitor.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2000-103668
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2002-50536
Patent Document 3: Japanese Patent No. 3509710
Each of the dielectric ceramic compositions proposed in Patent Documents 1, 2, and 3 can be used with high-frequency high-voltage alternating current or high-voltage direct current and has high reliability. However, it is predicted that trends toward miniaturization and higher capacitance of electronic devices lead to more stringent service conditions. Furthermore, it is also predicted that an improvement in reliability is further required. Thus, it is an urgent issue to ensure and improve reliability under such stringent service conditions.